The present invention relates generally to anti-skid brake control systems for normally over-riding manual operation of the brakes of a vehicle upon sensing a skid condition and, more particularly, relates to a short circuit detector in such a system for disabling the system when a short circuit condition is detected therein.
In electrical systems associated with vehicles there is a possibility of encountering a short circuit condition whereby a wire of such a system inadvertently becomes electrically coupled to the vehicle chassis or the like, for example, when a sharp object scrapes the insulation from a wire, and the short circuit usually will render one or more of the vehicle electrical systems inoperative. Continuation of the short circuit condition may drain the vehicle battery or other power supply and/or the unusually heavy current flow due to the short circuit may damage other electrical components of the short circuited system or other systems of the vehicle.
When such a short circuit condition affects a safety-related system of a vehicle, such as an anti-skid brake control system, the potential problems may become critical ones that interfere with safe vehicle operation. For example, a typical electrical system for controlling skid of a vehicle by over-riding manual operation of the vehicle brakes upon sensing a skid condition may dump or release via a conventional modulator valve part or all of the brake operating fluid pressure, such as air pressure, which the driver may be attempting manually to apply by foot operation of the brake pedal, in an automatic effort to allow the skidding vehicle wheels to regain traction on the roadway before maximum braking is tried again. In U.S. Pat. No. 3,917,359 such an anti-skid brake control system is described. In that patent there is also described a circuit to switch off or to disable the anti-skid control system, when an open circuit fault condition occurs in one of the sensors that detects wheel speed, thereby to return to the driver absolute control of the vehicle brake system without improper automatic over-ride interference by the faulty control system and, thus, assuring that a failure of the latter will be in a safe direction. Moreover, in another anti-skid control system the detection of a fault, for example, such an open circuit fault, effects both a latching out of the skid control mechanism from affecting the vehicle brakes and at the same time the energizing of an indicator device to provide a warning signal to the driver that a fault has been detected and that the anti-skid brake control system is presently inoperative. When the anti-skid brake control system of the latter type is operating normally, it usually provides a positive voltage at a warning terminal output, which may be coupled to the mentioned indicator to preclude its energization, and when a fault has been detected by such system a negative or relative ground voltage is provided on that warning terminal output to effect energization of the indicator device.
If the warning terminal output of the above-mentioned system were inadvertently short circuited to the vehicle chassis, for example, the energized indicator device would then incorrectly signify to the driver that the anti-skid brake control system was latched out and that full manual control of the vehicle brakes had been returned to the driver; whereas, in fact, except for the false indication or signaling, the anti-skid brake control system actually would be properly operating.